Pneumatic rubber tires conventionally have outer rubber sidewall layers which may be subject to considerable flexing and scuffing as well as atmospheric aging.
In practice, such outer, atmospherically exposed, visible tire rubber sidewall outer layers may age somewhat prematurely as a result of, for example, weather aging due to atmospheric conditions, fatigue cracking due to continual flexing under operating conditions, and abrasion due to scuffing.
Aging of the rubber sidewall outer layer due to weathering may be, for example, a result of exposure to ultraviolet light, ozone and/or high humidity. Antidegradants are conventionally mixed with the sidewall rubber to counteract or retard such effects.
Fatigue cracking of the sidewall rubber outer layer, when it occurs, is usually due to continual flexing of the tire sidewall as the tire is run under load at service conditions, particularly if it has become atmospherically aged.
Use of a low unsaturation EPDM based rubber composition has been proposed for a tire outer sidewall outer layer to resist its aging because of the resistance of EPDM rubber to atmospheric (e.g. ozone) weathering because of its low unsaturation content. For example, see U.S. Pat. No. 5,386,865.
However, rubber compositions comprised of carbon black reinforced homogeneous EPDM/conjugated diene-based elastomer (e.g. natural cis 1,4-polyisoprene rubber) blends which contain a relatively high EPDM content may be expected to typically exhibit a reduction in tear strength, or resistance to tear and a reduction in adhesion (e.g. reduction in adhesion to itself and to adjoining tire components comprised of high unsaturation conjugated diene-based elastomers), as compared to a natural rubber based rubber composition, as well as a reduction in flex fatigue resistance.
Further, carbon black reinforcement of an EPDM/conjugated diene-based elastomer (e.g. natural cis 1,4-polyisoprene rubber) blend is considered herein to be a challenge in a sense of the carbon black reinforcement being more compatible with the conjugated diene-based elastomer (e.g. natural rubber) then the EPDM rubber and thereby contained in a greater proportional amount and presenting a greater carbon black reinforcement capability in the natural rubber portion as compared to the EPDM portion.
A further challenge is presented in the sulfur curing of an EPDM/conjugated diene-based elastomer (e.g. natural rubber) blend in a sense of sulfur and sulfur vulcanization accelerators being typically more soluble in the conjugated diene-based elastomer (e.g. natural rubber) portion than the EPDM portion of the rubber composition and thereby contained in greater proportion in the conjugated diene-based elastomer (e.g. natural rubber) portion of the rubber composition. Such challenge is amplified in a sense of the EPDM containing significantly less unsaturation and therefore presenting a lesser sulfur crosslinked content.
Further, the sulfur cure rate, or sulfur vulcanization rate, of the EPDM/conjugated diene-based elastomer (e.g. natural rubber) blend is considered herein to be a challenge in a sense of the sulfur cure rate of the lower unsaturation EPDM rubber being significantly slower than the cure rate of the high unsaturation conjugated diene-based elastomer (e.g. natural rubber).
In addition, such EPDM/conjugated diene-based elastomer (e.g. natural rubber) based outer rubber sidewall layer may present significant sulfur cure rate compatibility challenges with adjoining tire components composed of a more normally faster sulfur curing high unsaturation conjugated diene-based rubber compositions.
Accordingly, in one aspect, sulfur curatives (e.g. sulfur vulcanization accelerators) may be considered for use in this invention:
(A) which have a greater solubility in the EPDM rubber than the high unsaturation conjugated diene-based elastomer (e.g. cis 1,4-polyisoprene rubber) to minimize diffusion of the sulfur curative(s) from the low unsaturation EPDM rubber into the high unsaturation conjugated diene-based rubber, such as for example, zinc octadecylisopropyldithiocarbamate, or
(B) which have a relatively high molecular weight to minimize diffusion of the sulfur curative(s) from the low unsaturation EPDM rubber into the high unsaturation conjugated diene-based rubber, or
(C) to bond with the EPDM rubber to minimize their migration to the high unsaturation conjugated diene-based elastomer phase of the rubber composition.
In practice, for this invention, it is proposed to phase mix a rubber composition comprised of EPDM and conjugated diene-based elastomers, particularly such elastomers which comprise cis 1,4-polyisoprene rubber and more particularly natural cis 1,4-polyisoprene rubber, with rubber reinforcing filler, such as for example rubber reinforcing carbon black, and with sulfur curatives.
In one aspect, such phase mixing is comprised of a sequential phase mixing of rubber reinforcing carbon black with an EPDM-rich rubber composition followed by mixing a conjugated diene-based elastomer (e.g. cis 1,4-polyisoprene rubber) therewith in order that the rubber reinforcing carbon black preferentially associates itself first and thereby primarily with the EPDM rubber before the conjugated diene-based elastomer (e.g. cis 1,4-polyisoprene rubber) is added.
By such sequential phase mixing process, the rubber reinforcing carbon black remains primarily associated with the EPDM rubber in the EPDM/conjugated diene-based elastomer (e.g. cis 1,4-polyisoprene rubber) composition and therefore promotes an enhancement of the overall carbon black reinforcement of the rubber composition as compared to a more simple homogeneous mixing of the EPDM rubber and conjugated diene-based rubber such as a cis 1,4-polyisoprene rubber and particularly natural cis 1,4-polyisoprene rubber.
In another aspect, such phase mixing is comprised of a parallel phase mixing of individual phase mixed rubber composition (which might be sometimes referred to as “Y-mixing”) in which a first rubber composition is prepared by blending the EPDM rubber with a majority of the rubber reinforcing carbon black together with which a majority, and perhaps all, of the sulfur curatives comprised of sulfur and organic sulfur vulcanization accelerator(s) and a second rubber composition is prepared by blending the conjugated diene-based rubber(s), such as for example the cis 1,4-polyisoprene rubber, (particularly natural rubber) with a minor amount of the rubber reinforcing carbon black and a minor amount, if any, of said sulfur curatives, followed by blending said first and second rubber compositions together in order that said sulfur curatives and a major portion of said rubber reinforcing carbon black associates themselves first, and thereby primarily with, the EPDM rubber in order to both minimize sulfur cure rate incompatibilities between the EPDM rubber portion and the conjugated diene rubber portion of the rubber composition and to maximize the carbon black reinforcement of the overall rubber composition.
For the purposes of this invention is the EPDM rubber is composed of at least two EPDM elastomers which are differentiated from each other.
For such purpose, a first EPDM elastomer is provided which is more elastomeric in nature by containing a higher non-conjugated diene content and thereby promotes a more rapid rate of sulfur curing, insofar as the EPDM elastomer is concerned, to aid in reducing the sulfur rate incompatibility between the EPDM rubber and conjugated diene-based elastomer(s), such as for example the natural cis 1,4-polyisoprene rubber.
For such purpose, second EPDM elastomer is provided which is less elastomeric in nature than the first EPDM elastomer by having a lower non-conjugated diene content. Its somewhat higher ethylene content (e.g. a higher ethylene/propylene ratio) may present the second EPDM elastomer as being somewhat being somewhat more self-reinforcing than the first EPDM elastomer in circumstances where it contains polyethylene-based crystalline domains at its higher ethylene content levels.
The use of the dual EPDM elastomer blend, then, is intended to promote mechanical strength and interfacial strength between the EPDM rubbers and conjugated diene-based rubbers, particularly the conjugated-diene based rubbers comprised of cis 1,4-polyisoprene rubber, particularly natural cis 1,4-polyisoprene rubber.
In practice, the non-conjugated diene for the EPDM terpolymer rubber may be selected from, or comprised of, for example, ethylidene norbornadiene, dicyclopentadiene or trans 1,4-hexadiene, with ethylidene norbornadiene being often preferred. Such EPDM terpolymer rubbers, in general, are well known to those having skill in such art.
In the description of this invention, the term “phr” relates to parts by weight of an ingredient per 100 parts by weight of rubber.
The terms “rubber” and “elastomer”, “cure” and “vulcanize”, and “compound” and “composition” may be used interchangeably unless otherwise indicated.